This invention relates to catalyst systems that facilitate the selective production of 1-octene in combination with a suitable polymerisation catalyst so that the in situ polymerisation of the formed 1-octene with the available ethylene feedstock takes place. Preferably, ethylene tetramerisation and polymerisation take place at the same time under the same reaction conditions.
With regard to the oligomerisation catalysts used in this application that are used for the production of 1-octene, the art does not teach a commercially successful process for the tetramerisation of ethylene to produce 1-octene selectively. Conventional ethylene oligomerisation technologies produce a range of α-olefins following either a Schulz-Flory or Poisson product distribution. By definition, these mathematical distributions limit the mass % of the tetramer that can be formed and make a distribution of products. In this regard, it is known from prior art (U.S. Pat. No. 6,184,428) that a nickel catalyst comprising a chelating ligand, preferably 2-diphenyl phosphino benzoic acid (DPPBA), a nickel compound, preferably NiCl2.6H2O, and a catalyst activator, preferably sodium tetraphenylborate, catalyse the oligomerisation of ethylene to yield a mixture of linear olefins containing 1-octene. The selectivity towards linear C8 α-olefins is claimed to be 19%. Similarly the Shell Higher Olefins Process (SHOP process, U.S. Pat. Nos. 3,676,523 and 3,635,937) using a similar catalyst system is reported to typically yield 11 mass % 1-octene in its product mixture (Chem Systems PERP reports 90-1, 93-6 and 94/95S12).
Ziegler-type technologies based on trialkylaluminium catalysts, independently developed by Gulf Oil Chemicals Company (Chevron, e.g. DE patent 1,443,927) and Ethyl Corporation (BP/Amoco, e.g. U.S. Pat. No. 3,906,053), are also commercially used to oligomerise ethylene to produce mixtures of olefins that reportedly contain 13-25 mass % 1-octene (Chem Systems PERP reports 90-1, 93-6, and 94/95S12).
The prior art also teaches that chromium-based catalysts containing heteroatomic ligands with both phosphorus and nitrogen heteroatoms selectively catalyse the trimerisation of ethylene to 1-hexene. Examples of such heteroatomic ligands for ethylene trimerisation include bis(2-diethylphosphino-ethyl) amine (WO 03/053891) as well as (o-methoxyphenyl)2PN(methyl)P(o-methoxyphenyl)2 (WO 02/04119). Both these catalyst systems and processes are very specific for the production of 1-hexene and only yield 1-octene as an impurity (typically less than 3 mass % of the product mixture as disclosed by WO 02/04119). The coordinating phosphorus heteroatoms in (o-methoxyphenyl)2PN(methyl)P(o-methoxyphenyl)2 (WO 02/04119) are spaced apart by one nitrogen atom. It is believed that the nitrogen atom does not coordinate, at least in the absence of an activator, with the chromium and that without any further electron donating atoms on the ligand that it is a bidentate system. Furthermore it is argued that the polar, or electron donating substituents in the ortho-position of the phenyl groups help form a tridentate system, which is generally believed to enhance selectivity towards 1-hexene formation as reiterated in WO 02/04119 in Chem. Commun., 2002, 858-859: “This has led us to hypothesise that the potential for ortho-methoxy groups to act as pendent donors and increase the coordinative saturation of the chromium centre is an important factor.” WO 02/04119 (Example 16) teaches the production of octenes using a trimerisation of olefins process and catalyst system. In this instance, 1-butene was co-trimerised with two ethylene molecules to give 25% octenes. However, the nature of these octenes was not disclosed and the applicant believes that they consist of a mixture of linear and branched octenes.
This application discloses the combination of highly selective tetramerisation cocatalysts for the production of 1-octene and the concomitant polymerisation of the tetramerised olefin with the ethylene feedstock. It has further been found that a chromium-based catalyst containing mixed heteroatomic ligands can be used to selectively tetramerise ethylene to 1-octene often in excess of 60 mass % selectivity. This high 1-octene selectivity cannot be achieved via conventional one-step ethylene oligomerisation technologies which at most yield 25 mass % 1-octene and the present invention confers the suitability of a selective 1-octene yielding catalyst for in situ polymerisation.
Examples of heterogeneous polymerisation processes that mention the polymerisation of in situ formed α-olefins to yield branched by-products of the polymerisation process are known in the art. However, few examples are available describing the combination of a heterogeneous polymerisation catalyst on a support and a homogeneous oligomerisation catalyst, or the combination of two homogeneous catalyst systems where one system produces an α-olefin and the other catalyst polymerises the olefin formed in situ as a comonomer with ethylene. Specific examples related to tandem oligomerisation and polymerisation catalysis involving a supported polymerisation catalyst include U.S. Pat. No. 5,331,070 and EP 0 417 477 in which a metal alkyl, a pyrrole-containing compound and a chromium salt supported on an inorganic support material polymerises an in situ generated α-olefin to yield a branched polyethylene.
Tandem catalysis involving heterogeneous polymerisation catalysts mostly does not involve the combination of a separate oligomerisation catalyst together with a supported polymerisation catalyst as proposed in this application. Due to the nature and geometry of the supported polymerisation catalyst, the formation of α-olefins through β-hydride elimination is possible to some extent, however the formation of polymers is thermodynamically and kinetically more favoured. After an α-olefin is produced it has a high probability to be incorporated into other growing polymer chains as the process of chain growth proceeds from the surface of the support material. In this way, polymers containing relatively low amounts of α-olefins are produced without significant control over the extent of α-olefin incorporation or the resultant polymer architecture.
Although examples of homogeneous tandem catalysis involving the combination of oligomerisation catalysts in tandem with suitable soluble polymerisation catalysts are rare, the recent patent literature such as U.S. Pat. Nos. 6,586,541, 6,555,631 and 6,586,550 specifically relate to the deliberate combination of oligomerisation catalysts, previously reported for their ability to produce α-olefins, combined with soluble polymerisation catalysts to produce branched polyolefins.
These patent examples centre on contacting both an oligomerisation catalyst and a polymerisation catalyst with a single feedstock namely ethylene. Both oligomers and polymers are produced in the same system with concurrent production of α-olefins and the polymerisation of the resultant α-olefins with ethylene. Most of the oligomerisation catalysts that convey novelty to these patents are oligomerisation catalysts that produce a distribution of α-olefins. These distributions are favoured towards a mixture of 1-butene, 1-hexene, 1-octene and 1-decene. Consequently, the structure of the polymers described in these patents exhibit patterns of incorporation according to 13C NMR analysis that reflect the distribution of α-olefins.
It is well-known in the open literature that control of reaction conditions such as pressure, temperature, feed gas make-up, comonomer content etc. result in polymers with desirable physical properties such as tear-strength, optical clarity, elasticity and many other physical properties deemed desirable by polymer end-users. In particular, the production of linear low-density polyethylene (LLDPE) sees the polymerisation of individual high purity α-olefins, such as 1-butene, 1-hexene and 1-octene, and ethylene to produce LLDPE suitable for films and sheets, blow molding, extrusion and wire and cable jacket material as well as rotational molding material. In none of the above examples is there disclosed a process relating to the selective production of 1-octene for in situ incorporation into polyethylene.
The combination of the highly selective ethylene tetramerisation catalyst system of the present disclosure with a suitable polymerisation catalyst can therefore result in a tandem process that produces polyethylene grades with high end-user specifications without the need of a separate facility or process to manufacture the 1-octene.
It will be understood that tandem oligomerisation and polymerisation catalysis includes in situ catalysis. In-situ catalysis means oligomerisation and polymerisation catalysis in the same reaction medium. The tandem oligomerisation and polymerisation catalysis may be in situ concurrent and/or in situ consecutive catalysis. The individual components of the oligomerisation and polymerisation catalysts may thus be added together simultaneously or sequentially, in any order, and in the presence or absence of monomer in a suitable solvent. Reaction mixture will be understood to include a reaction medium, reaction products and catalyst components. The reaction medium normally includes a solvent.